Methods and system for generating a waveform for transmitting data to a plurality of receivers and for decoding the received waveform

Abstract

Methods and a system are described for generating a waveform for transmitting data over a channel divided into a plurality of adjacent frequency subcarriers. One method includes receiving a plurality of data bits, each destined for a different receiver of a plurality of receivers. For each received data bit, the method further includes coding the data bit using a unique spreading code of a first set of spreading codes to generate a corresponding group of multiple copies of a data symbol. Additionally, the groups of data symbols, corresponding to the plurality of data bits, are interleaved to generate a sequence of interleaved data symbols, and the sequence of interleaved data symbols is mapped to the plurality of adjacent frequency subcarriers to generate a waveform symbol.

We claim: 
     
       1. A method for generating a waveform for transmitting data to a plurality of receivers over a channel divided into a plurality of adjacent frequency subcarriers, the method comprising:
 receiving a plurality of data bits, each destined for a different receiver of a plurality of receivers; 
 for each received data bit, coding the data bit using a unique spreading code of a first set of spreading codes to generate a corresponding group of multiple copies of a data symbol; 
 interleaving the groups of data symbols, corresponding to the plurality of data bits, to generate a sequence of interleaved data symbols; 
 applying a second set of spreading codes to the sequence of interleaved data symbols to differentiate which group of data symbols is destined for which receiver of the plurality of receivers; 
 mapping the sequence of interleaved data symbols to the plurality of adjacent frequency subcarriers to generate a waveform symbol, wherein the mapping comprises applying a modulation scheme to the sequence of interleaved data symbols. 
 
     
     
       2. The method of  claim 1 , wherein the sequence of interleaved data symbols is mapped to the plurality of adjacent frequency subcarriers such that no two copies of the data symbol within any group are mapped to directly adjacent frequency subcarriers. 
     
     
       3. The method of  claim 1 , wherein each spreading code, of the first set of spreading codes, comprises a spreading sequence of length M, and coding the received data bits comprises generating an MXN matrix of the groups of data symbols, wherein N equals the number of receivers in the plurality of receivers, and wherein each row of the matrix comprises the group of data symbols generated for a different one of the received data bits. 
     
     
       4. The method of  claim 3 , wherein the first set of spreading codes comprises a set of orthogonal time-domain spreading codes. 
     
     
       5. The method of  claim 4 , wherein the set of orthogonal time-domain spreading codes comprises a set of Walsh sequences. 
     
     
       6. The method of  claim 4 , wherein the sequence of interleaved data symbols is mapped to the plurality of adjacent frequency subcarriers such that at least a portion of the plurality of adjacent frequency subcarriers are non-orthogonal. 
     
     
       7. The method of  claim 3 , wherein M equals N. 
     
     
       8. The method of  claim 3 , wherein interleaving the groups of data symbols to generate the sequence of interleaved data symbols comprises reading the data symbols of the matrix column-by-column into the sequence of interleaved data symbols. 
     
     
       9. The method of  claim 1 , wherein the second set of spreading codes comprises a set of frequency-domain spreading codes that includes a set of complex pseudorandom noise sequences, wherein each complex pseudorandom noise sequence in the set has low correlation, and each complex pseudorandom noise sequence in the set is assigned to a different receiver in the plurality of receivers. 
     
     
       10. The method of  claim 1 , wherein mapping the sequence of interleaved data symbols to the plurality of adjacent frequency subcarriers comprises:
 applying a modulation scheme having an adjustable data rate to the sequence of interleaved data symbols to generate a plurality of modulation symbols; and 
 applying an inverse Fast Fourier Transform to the plurality of modulation symbols to generate the first waveform symbol. 
 
     
     
       11. The method of  claim 10 , wherein applying the modulation scheme having the adjustable data rate comprises applying a selected one of multiple quadrature amplitude modulation forms each characterized by a different data rate, wherein the one of the multiple quadrature amplitude modulation forms is selected based on conditions of the channel. 
     
     
       12. A system configured to generate a waveform for transmitting data to a plurality of receivers over a channel divided into a plurality of adjacent frequency subcarriers, the system comprising:
 a data spreader configured to receive a plurality of data bits each destined for a different receiver of a plurality of receivers, and, for each received data bit, to code the data bit using a unique data spreading code of a first set of data spreading codes to generate a corresponding group of multiple copies of a data symbol; 
 an interleaver coupled to the data spreader and configured to interleave the groups of data symbols, corresponding to the plurality of data bits, to generate a sequence of interleaved data symbols; 
 a modulator coupled to the interleaver and configured to apply a modulation scheme and an inverse discreet Fourier transform to the sequence of interleaved data symbols to map the sequence of interleaved data symbols to the plurality of adjacent frequency subcarriers to generate a waveform symbol; 
 a coding block coupled between the interleaver and the modulator and configured to apply a second set of spreading codes to the sequence of interleaved data symbols to differentiate which group of data symbols is destined for which receiver of the plurality of receivers. 
 
     
     
       13. The system of  claim 12 :
 wherein the data spreader is configured to code the plurality of received data bits using a set of orthogonal spreading codes as the first set of data spreading codes; and 
 wherein the modulator is configured to map the sequence of interleaved data symbols to the plurality of adjacent frequency subcarriers such that at least a portion of the plurality of adjacent frequency subcarriers are non-orthogonal. 
 
     
     
       14. The system of  claim 12 , wherein the coding block comprises a complex pseudorandom noise sequencer configured to apply, as the second set of spreading codes, a set of frequency-domain spreading codes that includes a set of complex pseudorandom noise sequences, wherein each complex pseudorandom noise sequence in the set has low correlation, and each complex pseudorandom noise sequence in the set is assigned to a different receiver in the plurality of receivers. 
     
     
       15. The system of  claim 14 , wherein each complex pseudorandom noise sequence in the set includes an in-phase pseudorandom noise sequence and a quadrature pseudorandom noise sequence, wherein the complex pseudorandom noise sequencer comprises:
 a first linear feedback shift register configured to generate a set of in-phase pseudorandom noise sequences for applying to the sequence of interleaved data symbols to generate an in-phase sequence of interleaved data symbols; and 
 a second linear feedback shift register configured to generate a set of quadrature pseudorandom noise sequences for applying to the sequence of interleaved data symbols to generate a quadrature sequence of interleaved data symbols; 
 wherein each complex pseudorandom noise sequence in the set has minimal correlation, and each complex pseudorandom noise sequence in the set is assigned to a different receiver in the plurality of receivers; 
 wherein the modulator is configured to apply to the in-phase and quadrature sequences of interleaved data symbols, as the modulation scheme, a selected one of multiple quadrature amplitude modulation orders each characterized by a different data rate, wherein the one of the multiple quadrature amplitude modulation orders is selected based on conditions of the channel. 
 
     
     
       16. The system of  claim 12 :
 wherein the data spreader is configured to generate a matrix having M rows of the data symbols, where M equals the number of receivers in the plurality of receivers, and where each row of the matrix comprises the group of data symbols generated for a different one of the received data bits; 
 wherein the interleaver is configured to read the data symbols of the matrix columnwise into the sequence of interleaved data symbols. 
 
     
     
       17. A method for decoding a waveform symbol having data for a plurality of receivers, the method comprising:
 receiving, by a first receiver from a transmitter over a channel divided into a plurality of adjacent frequency subcarriers, a waveform symbol carrying a different data bit for each of a plurality of receivers, including a first data bit for the first receiver; 
 determining a plurality of modulation symbols from the waveform symbol; 
 demapping the plurality of modulation symbols to generate a representation of at least one sequence of interleaved data symbols comprising interleaved groups of data symbols generated for the plurality of receivers; 
 decorrelating the representation of the at least one sequence of interleaved data symbols based on a first sequence allocated only to the first receiver of the plurality of receivers to extract only the representation of the group of data symbols generated for the first receiver; 
 despreading the representation of the extracted group of data symbols using a second sequence allocated only to the first receiver of the plurality of receivers to decode the first data bit for the first receiver. 
 
     
     
       18. The method of  claim 17 :
 wherein the demapping is based on a quadrature amplitude modulation order selected based on detected conditions of the channel; 
 wherein the first sequence is a complex pseudorandom noise sequence having low correlation to other complex pseudorandom noise sequences allocated to the remaining receivers in the plurality of receivers; 
 wherein the second sequence comprises a Walsh sequence that is orthogonal to other Walsh sequences allocated to the remaining receivers in the plurality of receivers. 
 
     
     
       19. The method of  claim 17 , wherein the despreading generates a set of soft bits, the method further comprising:
 applying a first function to the set of soft bits to generate a single soft bit for the first receiver; 
 applying a second function to the single soft bit to decode the first data bit for the first receiver. 
 
     
     
       20. The method of  claim 17  further comprising performing frequency offset compensation to the waveform symbol before applying a discreet Fourier transform to the waveform symbol, wherein the discreet Fourier transform is used in determining the plurality of modulation symbols from the waveform symbol.